1. Field of the Invention
The present invention generally relates to a braking system for a vehicle and, more particularly, to a braking system controlling braking force of an electric vehicle.
2. Description of the Related Art
Conventionally, a regenerative braking system provided on an electric vehicle has been known as disclosed in Japanese Laid-open Patent Application No 7-205800. The system has a motor as a power supply for the vehicle. The motor generates traction torque according to electric power supplied to the motor. Further the motor converts torque energy supplied from driving wheels to electric power.
The ability of the motor to act as an alternator depends on the strength of the magnetic field generated inside the motor and speed of a coil moving through the magnetic field. The speed of the coil is proportion to rotational speed of the driving wheels. Accordingly, the generative ability of the motor depends on the strength of the magnetic field and the rotational speed of the driving wheels. The motor changes the strength of the magnetic field according to an operating signal supplied to the motor. Therefore, the generative ability of the motor changes according to the operating signal.
When a driver of the vehicle requires braking force, the system supplies an appropriate hydraulic pressure to each wheel cylinder provided to each wheel and requires the motor to generate appropriate electric power. When the hydraulic pressure is supplied to the wheel cylinders, each wheel generates a braking force according to the hydraulic pressure. Hereinafter, the braking force is referred to as hydraulic braking force.
When the requirement is given to the motor, the motor generates an appropriate electric power alternating torque energy supplied from the driving wheels. The system supplies the electric power to a battery equipped to the vehicle. When the motor generates the electric power, braking torque according to the electric power is exerted on the driving wheels. Therefore, the driving wheels generate a braking force when the motor generates the electric power. Hereinafter the electric power generated by the motor is referred to as regenerative electric power. Further the braking force following to the electric power is referred to as regenerative braking force.
As discussed above, when braking force is required by the driver the system generates the hydraulic braking force at all wheels, generates the regenerative braking force at the braking wheels, and regenerates a part of the braking energy supplying the regenerative electric power to the battery. Accordingly, the system permits a highly energy efficient operation of the vehicle while generating a large braking force.
The energy efficiency of the vehicle is increased, as the regenerative braking force becomes greater. Thus, it is preferable to generate a maximum regenerative braking force when the driver requires a braking force. In the electric vehicle, the maximum regenerative braking force depends on rotational speed of the driving wheels, charging state of the battery and so on. The system calculates the maximum regenerative braking force based on the parameters and requires the motor to generate the maximum regenerative braking force when the driver requires a braking force. Therefore, according to the system, it is possible to make the vehicle highly energy efficient.
A braking characteristic of a vehicle is greatly affected by a ratio of the braking force of driving wheels and following wheels. When a braking force proportion to pedal-pressing force is generated at both of the driving wheels and the following wheels, a preferable braking characteristic is obtained. Hereinafter, the braking force, which is generated at each wheel and makes the braking characteristic preferable, is referred to as ordinary braking force.
The braking system discussed above generates the hydraulic braking force and the regenerative braking force at the driving wheels. Thus, the driving wheels generate a larger longitudinal force than the following wheels. The wheels generate a large lateral force, as the longitudinal force decreases. Therefore, the system controls the hydraulic braking force of the following wheels as the ordinary braking force and the hydraulic braking force of the driving wheels to a value obtained by subtracting the maximum regenerative braking force from the ordinary braking force. According to the control discussed above enough lateral force is obtained at both of the driving wheels and the following wheels for generating the maximum regenerative braking force.
However, in some cases, the maximum regenerative braking force is rapidly reduced according to the change of state of the vehicle. More particularly, in a case where the battery becomes fully charged, during the time regenerative electric power is supplied to the battery, for example, the maximum regenerative braking force is reduced rapidly. Further, in a case where the rotational speed of the driving wheels decreases, when the regenerative electric power is supplied to the battery, for example, the maximum regenerative braking force is reduced rapidly.
When the maximum regenerative braking force is reduced rapidly, the braking force of the driving wheels becomes much smaller than the braking force of the following wheels, until the reduced amount of the maximum regenerative braking force is taken up by the hydraulic braking force of the driving wheels. Therefore, total braking force exerted on the vehicle becomes momentarily lower than the braking force required by the driver.
As discussed above, the conventional braking system has some possibility of reducing the braking characteristic of the vehicle momentarily, while being useful to generate appropriate braking force at both of the driving wheels and the following wheels, thereby making the vehicle highly energy efficient.